The present invention, in some embodiments thereof, relates to an apparatus and method for detecting myocardial ischemia using analysis of high frequency components of an electrocardiogram, and more particularly but not exclusively to use of regions within the QRS complex known as reduced amplitude zones or RAZ.
ECG is used to measure the rate and regularity of heartbeats, as well as the size and position of the chambers, the presence of any damage to the heart, and the effects of drugs or devices used to regulate the heart.
ECG may be measured during rest (resting ECG) or when the heart is under stress (stress ECG).
Usually more than two electrodes are used, and they can be combined into a number of pairs (For example: left arm (LA), right arm (RA) and left leg (LL) electrodes form the three pairs LA+RA, LA+LL, and RA+LL). The output from each pair is known as a lead. Each lead looks at the heart from a different angle. Different types of ECGs can be referred to by the number of leads that are recorded, for example 3-lead, 5-lead or 12-lead ECGs. A 12-lead ECG is one in which 12 different electrical signals are recorded at approximately the same time and will often be used as a one-off recording of an ECG, traditionally printed out as a paper copy. 3- and 5-lead ECGs tend to be monitored continuously and viewed only on the screen of an appropriate monitoring device, for example during an operation or whilst being transported in an ambulance.
An ECG is the best way to measure and diagnose abnormal rhythms of the heart, particularly abnormal rhythms caused by damage to the conductive tissue that carries electrical signals, or abnormal rhythms caused by electrolyte imbalances. In a myocardial infarction (MI), the ECG can identify if the heart muscle has been damaged in specific areas, though not all areas of the heart are covered. Acute coronary syndrome (ACS) refers to any group of symptoms attributed to obstruction of the coronary arteries.
The ECG device detects and amplifies the tiny electrical changes on the skin that are caused when the heart muscle depolarizes and subsequently repolarizes during each heartbeat. At rest, each heart muscle cell has a negative charge, which causes the membrane potential, across its cell membrane. Decreasing this negative charge towards zero, via the influx of the positive cations, Na+ and Ca++, is called depolarization, which activates the mechanisms in the cell that cause it to contract. During each heartbeat, a healthy heart will have an orderly progression as a wave of depolarisation, that is triggered by the cells in the sinoatrial node, spreads out through the atrium, then passes through the atrioventricular node and finally spreads all over the ventricles. The progression is detected as waveforms in the voltage between two electrodes placed either side of the heart and may be displayed as a wavy line either on a screen or on paper. This display indicates the overall rhythm of the heart and weaknesses in different parts of the heart muscle.
A typical ECG tracing of the cardiac cycle (heartbeat) consists of a P wave, a QRS complex, a T wave, and a U wave which is normally visible in 50 to 75% of ECGs. The baseline voltage of the electrocardiogram is known as the isoelectric line. Typically the isoelectric line is measured as the portion of the tracing following the T wave and preceding the next P wave.
The standard ECG traces ignore, indeed usually filter out, high frequency components, for example signals above 100 Hz and in some case even lower thresholds such as 75 Hz or even 50 Hz. In general the noise level is such that high frequency components cannot be reliably isolated from a single ECG trace. In order to obtain high frequency components one typically needs to align ECG traces from successive heartbeats so that noise cancels.
One feature that appears from the high frequency component following alignment is the reduced amplitude zone or RAZ. The presence or absence of RAZ is discussed in the following patent disclosures:
U.S. Pat. No. 7,113,820 filed Jul. 12, 2001 and U.S. Pat. No. 7,539,535 filed Jan. 26, 2006 both disclose real time cardiac electrical data being received from a patient, manipulated to determine various useful aspects of the ECG signal, and displayed in real time in a useful form on a computer screen or monitor. The monitor displays the high to frequency data from the QRS complex in units of microvolts, juxtaposed with a display of conventional ECG data in units of millivolts or microvolts. The high frequency data are analyzed for their root mean square (RMS) voltage values and the discrete RMS values and related parameters are displayed in real time. The high frequency data from the QRS complex are analyzed with imbedded algorithms to determine the presence or absence of reduced amplitude zones, referred to herein as “RAZs”. RAZs are displayed as “go, no-go” signals on the computer monitor. The RMS and related values of the high frequency components are displayed as time varying signals, and the presence or absence of RAZs may be similarly displayed over time.
In U.S. Pat. No. 7,386,340 filed Mar. 26, 2003, a system for the diagnosis and monitoring of coronary artery disease, acute coronary syndromes, cardiomyopathy and other cardiac conditions is disclosed. Cardiac electrical data are received from a patient, manipulated to determine various useful aspects of the ECG signal, and displayed and stored in a useful form using a computer. The computer monitor displays various useful information, and in particular graphically displays various permutations of reduced amplitude zones and kurtosis that increase the rapidity and accuracy of cardiac diagnoses. The disclosure provides criteria for recognizing reduced amplitude zones that enhance the sensitivity and specificity for detecting cardiac abnormalities.
The above disclosures have in common that they decide whether or not a RAZ is present and draw their conclusions from that.